1. Field of the Invention
The present invention relates to lithium ion secondary batteries. More particularly, the present invention relates to lithium ion secondary batteries having a sealing tape attached to the outer circumference of the electrode assembly.
2. Discussion of Related Art
Recently, compact and light electrical and electronic apparatuses, such as mobile telephones, notebook computers, and camcorders, have been actively developed and produced. Battery packs are built in the portable electrical and electronic apparatuses so that the portable electrical and electronic apparatuses can operate without an additional power source. The built-in battery pack includes at least one battery for outputting a uniform voltage level to drive the portable electrical or electronic apparatus for a desired period of time.
Secondary batteries that can be charged and discharged have recently been used as the battery packs for economic reasons. Secondary batteries include Ni—Cd batteries, Ni-MH batteries, lithium metal batteries, and lithium ion batteries.
The operation voltage of lithium ion secondary batteries is commonly 3.6V, which is three times higher than the operation voltages of Ni—Cd batteries and Ni-MH batteries which are widely used as the power sources for portable electronic apparatuses. In addition, the energy density per unit weigh of lithium ion secondary batteries is high. Therefore, use of lithium ion secondary batteries has rapidly increased.
In lithium ion secondary batteries, lithium based oxides are used as the positive electrode active materials, and carbon materials are used as the negative electrode active materials. In general, lithium ion secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries depending on the kind of electrolyte. Batteries using liquid electrolytes are referred to as lithium ion batteries and batteries using polymer electrolytes are referred to as lithium polymer batteries. Also, lithium ion secondary batteries can take various shapes, such as cylinders, polygons, and pouches.
In general, a cylindrical lithium ion secondary battery includes an electrode assembly having a positive electrode plate coated with positive active materials, a negative electrode plate coated with negative active materials, and a separator positioned between the positive electrode plate and the negative electrode plate. The separator prevents the positive electrode plate and the negative electrode plate from short-circuiting and permits movement of only lithium ions. The electrode assembly is wound and placed in a case together with the electrolyte, and a cap assembly seals the case.
A sealing tape for supporting, protecting, and insulating the electrode assembly is wound around the outer circumference of the electrode assembly in the lithium ion secondary battery. The sealing tape is commonly formed of polyolefin based materials such as polyethylene (PE), polypropylene (PP), and polyimide (PI). However, these materials are barely wetted by the electrolyte and the electrolyte is barely diffused into the materials (the materials have a weak affinity for the electrolyte). As a result, the capillary phenomenon, which generates a minute gap between the case and the electrode assembly during impregnation of the electrolyte, is prevented. Furthermore, as the capacity of the battery increases, the density of the electrode assembly increases. Therefore, the external diameter of the electrode assembly increases and the space between the electrode assembly and the case decreases, making impregnation of the electrolyte more difficult.
In a cylindrical lithium ion secondary battery, before inserting the electrode assembly into the case, a lower insulating plate is inserted to insulate the electrode assembly and the case from each other. Also, after inserting the electrode assembly and before sealing the case with the cap assembly, an upper insulating plate is inserted to insulate the electrode assembly and the cap assembly from each other.
Since the upper and lower insulating plates are commonly formed of polyethylene (PE) or polypropylene (PP), the upper and lower insulating plates are barely wetted by the electrolyte and the electrolyte is barely diffused into the upper and lower insulating plates (the upper and lower insulating plates have weak affinities for the electrolyte). Therefore, the electrode assembly is prevented from being sufficiently impregnated with the electrolyte. Furthermore, as the capacity of the battery increases, the density of the electrode assembly increases. As a result, the external diameter of the electrode assembly increases. When the external diameter of the electrode assembly increases, the space between the case and the electrode assembly decreases, making impregnation of the electrolyte more difficult.